Ethylene cracking furnace

ABSTRACT

The present disclosure provides an ethylene cracking furnace, comprising at least one radiant section provided with a bottom burner and/or a side burner, and at least one set of radiant coil arranged along a longitudinal direction of the radiant section. The radiant coil is an at least two-pass coil having an N−1 structure, wherein N is preferably a natural number from 2 to 8. A manifold is arranged at an inlet end of a downstream tube of said at least two-pass coil, and an outlet end of each upstream tube of said at least two-pass coil is connected to the manifold through a curved connector. The arrangement according to the present disclosure can effectively reduce the expansion differences between the upstream tubes and the downstream tubes, and therefore reduce the stress caused thereby. Consequently, bending of the radiant coil can be avoided, thereby extending the service life of the radiant coil.

FIELD OF THE INVENTION

The present disclosure relates to the petrochemical engineering field,and more specifically, to a radiant coil structure of an ethylenecracking furnace used in petrochemical engineering.

BACKGROUND OF THE INVENTION

The ethylene cracking techniques used in petrochemical ethyleneequipments mainly include those developed by LUMMUS Co. (USA), Stone &Webster Co. (USA), Kellog & Braun Root Co. (USA), Linde Co. (Germany),Technip KTI Co. (Netherlands), and the CBL cracking furnace developed byChina Petrochemical Corporation.

FIG. 1A shows a typical ethylene cracking furnace 10, which comprises aradiant section 11, a convective section 13, and a flue section 12located between the radiant section 11 and the convective section 13.Within the radiant section 11, a radiant coil 14 is provided in thecentral plane P of the radiant section 11 along the longitudinaldirection thereof. In addition, the radiant section 11 is furtherprovided with bottom burners 15 and/or side burners 16 for heating.Moreover, the ethylene cracking furnace 10 further comprises a transferline exchanger 17, a high-pressure steam drum 18, and an induced draftfan 19, etc.

To significantly reduce the feedstock consumption, maintain a suitablerun length, and have a good feedstock flexibility, nowadays a two-passhigh-selectivity radiant coil with or without branches of variablediameters is used. The first-pass tube of the radiant coil is of a smalldiameter. Therefore, a quick temperature rise can be achieved since thespecific surface area of a small-diameter tube is relatively large. Thesecond-pass tube is of a large diameter, in order to reduce theinfluences on coking sensitivity. The two-pass radiant coil can beconfigured as 1-1 type (U type), 2-1 type, 4-1 type, 6-1 type coil, etc.

A two-pass 1-1 type coil structure, which can be matched to transferline exchanger(s), is of a large specific surface area and goodmechanical performance. The run length thereof, however, is slightlyshort.

For an N−1 (N>1) type coil structure, the number of tubes in the firstpass is N times as more as the number of the tubes in the second pass.Therefore, the N tubes in the first pass need to be combined into onetube before being connected to a corresponding second-pass tube. EP1146105 discloses a cracking furnace having a two-pass 2-1 type coilstructure. As indicated in FIG. 1B, a two-pass radiant coil comprisesfirst-pass tubes 51 (16 tubes) and second-pass tubes 52 (8 tubes)perpendicularly arranged in an inner chamber of a radiant section. Allthese tubes are located in one common plane, with all the first-passtubes 51 arranged together, and all the second-pass tubes 52 arrangedtogether, wherein every two first-pass tubes 51 are combined into onetube by a Y-shaped manifold 53 at a lower portion of the first-passtubes 51 before being connected to a second-pass tube 52 via twoS-shaped elbows 54 and a U-shaped elbow 55.

CN 1067669 discloses a cracking furnace having a two-pass 6-1 type coilstructure, which includes 6 first-pass tubes, and one second-pass tube.Similarly, these 6 first-pass tubes are first combined into one tube viaa rigid manifold arranged in a lower portion thereof, and then areconnected to the second-pass tube.

In the above structures, since the number of the first-pass tubes is aplurality of times higher than the number of the second-pass tubes, whenthe coil is heated to expand, the second-pass tubes first expanddownward, and then the first-pass tubes are dragged by the second-passtubes to move downward also, wherein the first-pass tubes are easilybent because they are deformed under different forces. The rigidity ofthe manifold connected in the lower portion of the first-pass tubesprevents expansion differences thereof from being absorbed by anS-shaped tube (if any), rendering the coil easily being bent. Hence, themechanical performance of the coil is reduced, thereby shortening theservice life of the coil and the run length of the cracking furnace.

SUMMARY OF THE INVENTION

To overcome the technical defects existing in the prior art, the presentdisclosure discloses a new ethylene cracking furnace having a two-passor multi-pass radiant coil, wherein a special arrangement structure ofthe radiant coil can reduce bending of the coil, thereby improving themechanical performance of the coil, extending service life thereof, andprolonging the run length of the cracking furnace.

According to the present disclosure, it provides an ethylene crackingfurnace, comprising at least one radiant section, which is provided witha bottom burner and/or a side burner, and at least one set of radiantcoil arranged along a longitudinal direction of the radiant section,wherein the radiant coil is an at least two-pass coil having an N−1structure, N preferably being a natural number from 2 to 8; and whereina manifold is arranged at an inlet end of a downstream tube of said atleast two-pass coil, and an outlet end of each upstream tube of said atleast two-pass coil is connected to the manifold through a curvedconnector.

In the text of the present disclosure, the term “coil having an N−1structure” means that in two adjacent passes of tubes, for eachdownstream tube there are N corresponding upstream tubes. It is easilyunderstood, in a two-pass coil having an N−1 structure, a manifoldtherein can have N input ends and one output end. According to onepreferred embodiment, the manifold is in the form of an invertedlyY-shaped pipe having N input ends and one output end, N equaling 2 or 4.When N equals 4, every two upstream tubes are first combined togethervia one Y-shaped pipe element before being connected to a curvedconnector. According to another embodiment, the manifold is in the formof a palm-like pipe having a plurality of input ends and one output end.In a coil having more than two passes, N−1 indicates N input ends andone output end, with all connection manners of a two-pass coil having anN−1 structure capable of being applied therein.

In one preferred embodiment, the radiant coil is a two-pass coil,wherein the upstream tube is a first-pass tube, while the downstreamtube is a second-pass tube. In another embodiment, the radiant coil is amulti-pass coil having more than two passes, wherein the upstream tubesare odd-number ones such as a first-pass tube, a third-pass tube, etc.,while the downstream tubes are even-numbered ones such as a second-passtube, a fourth-pass tube, etc.

According to one embodiment, the upstream tubes are divided into twogroups each with the same number of tubes respectively arranged at twosides of the downstream tube, and all of the upstream tubes and thedownstream tube are arranged in a common plane.

According to one embodiment, the curved connector comprises a U-shapedelbow and an S-shaped elbow, of which one connects to a lower portion ofa corresponding upstream tube, and the other connects to an inlet end ofthe manifold. It should be noted that the curved connector of thepresent disclosure can “connect” to the tube or manifold either directlyor indirectly via a transition pipe, which can be selected asspecifically required. In some preferred embodiments, the tube diameterof the curved connector equals the tube diameter of the upstream tube,which, for example, is especially suitable when N equals 2, or when N islarger than 2 and the manifold is in the form of a palm-like pipe.

According to one embodiment, viewed from a top view of the radiant coil,with respect to the downstream tube, corresponding S-shaped elbows arein parallel with each other, and/or corresponding U-shaped elbows arearranged in one and the same line. Preferably, all the S-shaped elbowsare parallel with one another. Alternately, all the S-shaped elbows aredivided into a plurality of groups, with all S-shaped elbows in eachgroup in parallel with one another.

According to one embodiment, the upstream tubes are divided into twogroups each with the same number of tubes respectively arranged at twosides of the downstream tube. In this embodiment, however, the plane inwhich the upstream tubes arranged at one side of the downstream tube arelocated and the plane in which the upstream tubes arranged at the otherside of the downstream tube are no longer arranged in a common planewith the downstream tube. Instead, with respect to the plane in whichthe downstream tube is located, the plane in which the upstream tubesarranged at one side of the downstream tube are located is in mirrorrelationship with the plane in which the upstream tubes arranged at theother side of the downstream tube are located. In one alternativeembodiment, the upstream tubes arranged at one side of the downstreamtube, the upstream tubes arranged at the other side of the downstreamtube, and the downstream tube are respectively in three planes parallelwith one another.

According to the present disclosure, the upstream tubes can be allarranged at one and the same side of the downstream tube with all theupstream tubes and the downstream tube positioned in a common plane.According to one embodiment, the curved connectors of two adjacentupstream tubes are respectively located at two sides of the plane inwhich the tubes are located. In one embodiment, the upstream tubes donot have a common plane with the downstream tube, but are respectivelyarranged in two parallel planes, which is in parallel with the plane inwhich the downstream tube is arranged. In another embodiment, theupstream tubes are respectively arranged in two planes in mirrorrelationship with each other with respect to the plane in which thedownstream tube is located.

Comparing with the prior art, the present disclosure brings about thefollowing advantageous technical effects:

(1) Since the first-pass tube is combined with the second-pass tube atthe lower portion thereof, and S-shaped elbows and U-shaped elbows areused, the stress caused by expansion differences among the first-passtubes that exist in 2-1 type, 4-1 type, and other types of coils can beeffectively reduced. Consequently, bending of the radiant coil can beavoided, thereby extending the service life of the radiant coil.

(2) The S-shaped elbows and U-shaped elbows of the upstream tubes havesmaller tube diameters when the upstream tubes are combined at the lowerportion the downstream tube than when the upstream tubes are combined atthe lower portion of the upstream tubes. Therefore, the upstream tubeshave better flexibility, which facilitates absorption of heat expansiondifferences in two adjacent passes of tubes, thus avoiding bending ofthe tubes and finally extending service life of the radiant coil.

(3) A small tube diameter of the first-pass tube results in a highspecific surface area thereof. Therefore, when the first-pass coil isextended, the specific surface area of the whole coil would beincreased, which facilitates extension of the run length of the crackingfurnace at the same cracking depth, and improves olefin yield at thesame run length.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A shows a typical ethylene cracking furnace according to the priorart;

FIG. 1B shows a typical two-pass 2-1 type coil structure according tothe prior art;

FIGS. 2A, 2B, and 2C respectively show a front view, a top view, and aside view of one embodiment of a two-pass 2-1 type coil structureaccording to the present disclosure, wherein first-pass tubes aredivided into two groups with the same number of tubes in each grouprespectively arranged at two sides of a second-pass tube;

FIGS. 3A, 3B, and 3C respectively show a front view, a top view, and aside view of another embodiment of the two-pass 2-1 type coil structureaccording to the present disclosure, wherein all the first-pass tubesare arranged at one and the same side of the second-pass tube;

FIGS. 4A, 4B, and 4C respectively show a front view, a top view, and aside view of one embodiment of a two-pass 4-1 type coil structureaccording to the present disclosure;

FIGS. 5A to 7C show front views, top views, and side views of threevariations of the two-pass 2-1 type coil structure according to thepresent disclosure, wherein the first-pass tubes are divided into twogroups with the same number of tubes in each group respectively arrangedat the two sides of the second-pass tube, or all the first-pass tubesare arranged at one and the same side of the second-pass tube;

FIGS. 8A to 10C show front views, top views, and side views of threevariations of the two-pass 2-1 type coil structure according to thepresent disclosure, wherein all the first-pass tubes are arranged at oneand the same side of the second-pass tube; and

FIGS. 11A to 11C respectively show a front view, a top view, and a sideview of one variation of the two-pass 4-1 type coil structure accordingto the present disclosure.

In the accompanying drawings, the same component or structure isindicated by the same reference sign.

DETAILED DESCRIPTION OF THE EMBODIMENTS

In the following the present disclosure will be discussed in detailswith reference to the accompanying drawings. It should be noted that thepresent disclosure aims to provide improvements on radiant coil in theradiant section of the ethylene cracking furnace. Other structures inthe ethylene cracking furnace, such as the convective section, thetransfer line exchanger and the like, are already known in the priorart. For example, the transfer line exchanger suitable for the presentdisclosure can be a double-coil transfer line exchanger (such as alinear transfer line exchanger, U-type transfer line exchanger, and thefirst level of a two-level transfer line exchanger, etc.), conventionalboiler, etc. Moreover, the two-pass radiant coil of the presentdisclosure can be mainly suitable for cracking liquid material, but alsosuitable for cracking gas material. In contrast, the multiple-passradiant coil of the present disclosure can be mainly suitable forcracking gas material, but also suitable for cracking liquid material.In addition, both of the two-pass radiant coil and the multiple-passradiant coil of the present disclosure can be used in building newcracking furnaces or reconstructing existing cracking furnaces. Theseare known to one ordinarily skilled in the art, and thus their detailsthereof are omitted here.

FIGS. 2A, 2B, and 2C illustrate a first embodiment according to thepresent disclosure, which involves a two-pass 2-1 type coil structure.As shown in the Figures, the two-pass 2-1 type coil according to theembodiment comprises two first-pass tubes 1 and one second-pass tube 2.The front view, i.e., FIG. 2A indicates that said two first-pass tubes 1are respectively arranged at two sides of the second-pass tube 2.Moreover, the three tubes have three center lines positioned in a commonplane P (see FIG. 2B).

According to the present disclosure, a lower end (i.e., an input end) ofthe second-pass tube 2 is provided with a manifold 3, which is used forcombining the two first-pass tubes 1 and connecting the same to thesecond-pass tube 2. In the specific embodiment, the manifold 3 is in theform of an invertedly U-shaped pipe, i.e., having two input ends and oneoutput end, wherein the output end is connected to the lower end of thesecond-pass tube 2. The two first-pass tubes 1 are respectivelyconnected to the two input ends of the manifold 3 via two respectivecurved connectors (each consisting of an S-shaped elbow 5 and a U-shapedelbow 4) arranged at lower ends of the two first-pass tubes 1 (i.e.,output ends thereof). It can be easily understood, for an N−1 type coilstructure (N>2), the manifold can be designed to have N input ends andone output end, i.e., in the form of a palm. In addition, the curvedconnector can be connected to the two input ends of the manifold 3 via atransition pipe to satisfy the requirements of process and mechanicaldesign. In one specific embodiment, the transition pipe, which can be astraight pipe or an elbow, has the same tube diameter as the curvedconnector.

With the arrangement of the manifold 3 as a rigid connecting structureat the lower end of the second-pass tube 2 instead of at the lower endof the first-pass tubes 1, stress caused by the expansion differencesbetween the first-pass tubes 1 and the second-pass tube 2 under heating,and stress unbalances caused by expansion differences between thefirst-pass tubes 1 can be absorbed by the S-shaped elbow 5 and theU-shaped elbow 4 arranged at the lower end of the first-pass tubes 1.Hence, deformation is reduced, thus extending the service life of thecoil.

Furthermore, according to the present disclosure, the S-shaped elbow 5and the U-shaped elbow 4 are connected to the lower end of thefirst-pass tubes 1 and have the same tube diameter as the first-passtubes 1, thereby essentially extending the length of the first-passtubes 1. Such being the case, the specific surface area of the tubes isincreased, which is advantageous for extending the run length of thecracking furnace under the same cracking depth, and for improvingproduct yield under the same run length of the cracking furnace.Besides, because the curved connector has the same tube diameter as thefirst-pass tubes, the flexibility thereof is improved, which facilitateselimination of thermal stress, thereby reducing deformation of the tubesand extending service life thereof.

Advantageously, the S-shaped elbow 5 and U-shaped elbow 4 connected tothe lower end of the first-pass tube 1 which is arranged at a left sideof the second-pass tube 2 (see FIG. 2A), and the S-shaped elbow 5 andU-shaped elbow 4 connected to the lower end of the other first-pass tube1 which is arranged at a right side of the second-pass tube 2 (see FIG.2A) are respectively located at two sides of the plane P (see FIGS. 2Band 2C). This arrangement facilitates more homogeneous absorption ofdeformation caused by the thermal stress, thus further reducing thetemperature on the surface of the tubes and extending the service lifethereof.

In one preferred embodiment, as indicated in FIG. 2B, the top view, therespective S-shaped elbows 5 of the two first-pass tubes 1 are inparallel with each other, while the respective U-shaped elbows 4 of thetwo first-pass tubes 1 are in one and the same line. More preferably,with respect to the center line of the second-pass tube 2, the S-shapedelbow 5 and U-shaped elbow 4 of the first-pass tube 1 located at oneside of the plane P are in 180° rotation symmetry with the S-shapedelbow 5 and U-shaped elbow 4 of the first-pass tube 1 located at theother side of the plane P.

Moreover, as required in the process or mechanical design, a straightpipe of certain length and of the same tube diameter as the first-passtubes can be provided between the manifold 3 and the curved connector.

According to one variation of the first embodiment, the first-pass tubes1 and the second-pass tube 2 can be arranged at different planes,wherein the curved connector can merely comprises the U-shaped elbow 4,while the S-shaped elbow 5 can be omitted.

Other embodiments according to the present disclosure will be explainedin the following. For the sake of simplicity, only features orcomponents that are different from those in the embodiment as explainedabove and the functions thereof will be discussed, while the samefeatures or components or the functions thereof will not be repeated.

FIGS. 3A, 3B, and 3C show a second embodiment according to the presentdisclosure. The second embodiment distinguishes from the firstembodiment in that both of the two first-pass tubes 1 are arranged atone and the same side of the second-pass tube 2 (see the front view FIG.3A). This arrangement can also realize the advantages as stated in thefirst embodiment, and is applicable in some cracking furnaces ofspecific structures. In the second embodiment, the S-shaped elbow 5 andU-shaped elbow 4 connected to the lower end of one of the first-passtubes 1, and the S-shaped elbow 5 and U-shaped elbow 4 connected to thelower end of the other of the first-pass tubes 1 are still respectivelyarranged at the two sides of the plane P in which all the three tubesare located (see FIGS. 3B and 3C).

In one embodiment, viewed form a side view, a group of the S-shapedelbow 5 and U-shaped elbow 4 is in minor relationship with another groupof the S-shaped elbow 5 and U-shaped elbow 4 with respect to the plane P(see FIG. 3C). In one embodiment not shown, however, both groups ofcurved connectors may not be in mirror relationship with each other, inorder to ensure the same length and weight between the elbows at the twosides.

Similarly, when the first-pass tubes 1 and the second-pass tube 2 arenot arranged in a common plane, the curved connector can only comprisethe U-shaped elbow 4, while the S-shaped elbow 5 can be omitted.

FIGS. 4A, 4B, and 4C show a third embodiment according to the presentdisclosure. The third embodiment is different from the first embodimentin that the third embodiment involves a two-pass 4-1 type coilstructure. As demonstrated by the Figures, both sides of the second-passtube 2 are provided with two first-pass tubes 1. The two first-passtubes 1 in either side are first combined into one pipe via a manifold6, then connected to the S-shaped elbow 5 and the U-shaped elbow 4, andfinally connected to the manifold 3 positioned at the lower end of thesecond-pass tube 2. In the embodiment, the manifold 6 is in the form ofa Y-shaped pipe element having two input ends and one output end. Inaddition, according to the requirements in the process and mechanicaldesign, the two first-pass tubes 1 in either side can first be combinedinto one pipe via one manifold 6, then connected to the S-shaped elbow 5and the U-shaped elbow 4 by connecting to one straight pipe, and finallyconnected to the manifold 3 arranged at the lower end of the second-passtube 2 via one transition pipe (i.e., a straight pipe or an elbow).

It can be easily understood that in one embodiment not shown, themanifold 6 can be omitted. Meanwhile, the manifold 3 can be modified tohave four input ends and one output end. In this case, the fourfirst-pass tubes 1 can be directly connected to the four input ends vianecessary elbows (i.e., U-shaped elbows 4 and S-shaped elbows 5), or viaa transition pipe (i.e., a straight pipe or an elbow).

FIGS. 5A, 5B, and 5C show a fourth embodiment according to the presentdisclosure. The fourth embodiment is still a two-pass 2-1 type coilstructure, which is designed in the same way as the first embodimentexcept that it comprises 8 second-pass tubes 2 arranged together side byside, and 16 first-pass tubes 1 which are divided into two groups with 8tubes in each group respectively arranged at the two sides of thesecond-pass tubes 2. The structure of the fourth embodiment isequivalent to a structure including 8 coils of the first embodimentsarranged together in parallel with one another. As shown in FIG. 5B, allthe 16 S-shaped elbows 5 are in parallel with one another. Furthermore,for each second-pass tube 2, the corresponding two U-shaped elbows 4 areplaced in one and the same line. Preferably, the corresponding U-shapedelbows 4 of each second-pass tube 2 are in parallel with one another.

In addition, preferably, at the two sides of the plane P, all connectingareas of the S-shaped elbows 5 and the U-shaped elbows 4 are located ina common plane Q, which is in parallel with the plane P.

FIGS. 6A, 6B, and 6C show a fifth embodiment according to the presentdisclosure. The fifth embodiment is substantially the same as the fourthembodiment except that not all the 16 S-shaped elbows 5 are in parallelwith one another. Instead, they are divided into several groups and allelbows in a group are in parallel with one another. As indicated in theFigures, the S-shaped elbows 5 are grouped with an outer elbow and aninner elbow, and the two S-shaped elbows 5 in each group are parallelwith each other.

FIGS. 7A, 7B, and 7C show a sixth embodiment according to the presentdisclosure. The sixth embodiment is substantially the same as the fourthembodiment except that the first-pass tubes 1 are not arranged to have acommon plane with the second-pass tube 2. As illustrated in FIG. 7C, theside view, a plane M in which eight first-pass tubes 1 are located atone side of the second-pass tube 2, and a plane M′ in which the othereight first-pass tubes 1 are located at the other side of thesecond-pass tube 2, form an acute angle respectively with respect to theplane P in which the second-pass tube 2 is located. Preferably, theplanes M and M′ are in mirror relationship with respect to the plane P.In addition, as shown in FIG. 7B, the top view, each of the first-passtubes 1 has an axis line L perpendicular to the plane P in which thesecond-pass tube 2 is located. It can be easily understood, in onespecific embodiment, the planes M, M′ can be in parallel with the planeP. That is, either the plane M or M′ defines an angle of zero with theplane P. Furthermore, it would easily occur to one skilled in the artthat this structure is applicable to any cases in which all thefirst-pass tubes are positioned at one and the same side of thesecond-pass tube 2 (for example in the second embodiment of the presentdisclosure).

FIGS. 8A, 8B, and 8C show a seventh embodiment according to the presentdisclosure. The seventh embodiment is substantially the same as thesecond embodiment except that it comprises five second-pass tubes 2arranged together side by side, and 10 first-pass tubes 1 arranged atone and the same side of the second-pass tubes 2. The structure of thisembodiment is equivalent to five coils as illustrated in the firstembodiment arranged together in parallel with one another. As shown inFIG. 8B, the S-shaped elbows 5 and U-shaped elbows 4 connected to thelower end of the first-pass tubes are staggered with each other withrespect to the plane P in which the tubes are located, i.e., theS-shaped elbow 5 and U-shaped elbow 4 connected to a first tube of thefirst-pass tubes are arranged at one side of the plane P (an upperportion in the top view), while the S-shaped elbow 5 and U-shaped elbow4 connected to a second tube of the first-pass tubes are arranged at theother side of the plane P (a lower portion in the top view), so on andso forth. Besides, all the S-shaped elbows 5 at the upper portion of thetop view are in parallel with one another, and all the U-shaped elbows 4thereof are also in parallel with one another. And all the S-shapedelbows 5 at the lower portion of the top view are in parallel with oneanother, and all the U-shaped elbows 4 thereof are also in parallel withone another.

Additionally, in this embodiment, viewed from the side view (see FIG.8C), the S-shaped elbows 5 and U-shaped elbows 4 respectively arrangedat the two sides of the plane P are in mirror relationship with eachother with respect to the plane P. In one embodiment not shown, however,the side projections thereof are not in symmetry in order to ensure thesame pipe length of the two curved connectors connected to one and thesame manifold.

FIGS. 9A, 9B, and 9C show an eighth embodiment according to the presentdisclosure. The eighth embodiment is substantially the same as theseventh embodiment except that the lower end of the first-pass tube 1 isfirst connected to the U-shaped elbow 4, then to the S-shaped elbow 5,and finally to the manifold 3. That is, the layout order of the U-shapedelbow 4 and S-shaped elbow 5 are different from that in any one of thepreceding embodiments. Preferably, the S-shaped elbows 5 respectivelyarranged at the two sides of the plane P in which the tubes are locatedare in mirror relationship with respect to the plane P in the top view.Still preferably, the pipe length of the connector connecting to thefirst-pass tube is the same as that of connecting to the second-passtube (see FIG. 9B).

FIGS. 10A, 10B, and 10C show a ninth embodiment according to the presentdisclosure. The ninth embodiment is substantially the same as the eighthembodiment except that all the U-shaped elbows are the same as oneanother, and the S-shaped elbows respectively positioned at the twosides of the plane P in which the tubes are located are not in mirrorrelationship with respect to plane P.

FIGS. 11A, 11B, and 11C show a tenth embodiment according to the presentdisclosure. The tenth embodiment, which is a two-pass 4-1 type coilstructure, is substantially the same as the first embodiment except thatit comprises four second-pass tubes 2 arranged together in parallel withone another, and 16 first-pass tubes 1 which are divided into two groupseach group with eight tubes respectively arranged at the two sides ofthe second-pass tubes 2. The structure of this embodiment is equivalentto four coils of the third embodiment arranged together in parallel withone another.

According to the present disclosure, an inner diameter of the first-passtube 1 can be in the range from 40 to 65 mm, an inner diameter of thesecond-pass tube can be in the range from 55 to 130 mm, and an innerdiameter of the connector connecting the first-pass and the second-passtubes can be in the range from 40 to 90 mm Furthermore, the length ofthe first-pass tube 1 generally can be selected as within the range from8 to 18 m, while the length of the second-pass tube 2 can be selectedwithin the range from 6 to 14 m. The above parameters, and otherparameters concerning the length and inner diameter of tubes andconnectors are not limited in the above ranges but can be selected asspecifically required, which is well known by one skilled in the art.

In one preferred embodiment, an intensified heat transfer member, suchas the twisted tube as disclosed in CN 1260469, can be further providedin the radiant coil structure, in order to facilitate absorption ofradiant heat.

Although the cracking furnace of the present disclosure is exemplarilydescribed with the two-pass radiant coil structure, it however beunderstood that the present disclosure can also be used in a radiantcoil structure having more than two passes. For example, in an 8-4-2-1type four-pass coil structure, a manifold can be provided at a lower endof a second-pass or a fourth-pass tube. One skilled in the art wouldeasily think of the above after reading the present disclosure.

Moreover, although in the foregoing the present disclosure is describedwith reference to one set of radiant coil arranged in one crackingfurnace, it can be understood that a plurality of sets of radiant coilscan be arranged in one single cracking furnace, dependent on the actualrequirements. When one cracking furnace is provided with a plurality ofradiant coils as described in the above embodiments, the radiant coilscan be arranged in sequence. Alternatively, the plurality of radiantcoils can be arranged in the form of manifolds. In this case, the coilsshould be arranged in a mirror-symmetric way.

Although the present disclosure is described in details with referenceto some embodiments, it would be apparent to one skilled in the art thatmodifications and variations may be made to somefeatures/components/structures of the present disclosure withoutdeparting from the spirit or scope of the invention. In particular, thefeatures disclosed in one embodiment can be combined with thosedisclosed in other embodiments in arbitrary ways unless the combinationsmay cause conflicts. It is intended that the present disclosure coversall the modifications and variations thereof provided they come withinthe scope of the appended claims and their equivalents.

The invention claimed is:
 1. An ethylene cracking furnace, comprising:one or more radiant sections, wherein at least one radiant sectioncomprises a bottom burner and/or a side burner, and one or more ofradiant coils arranged along a longitudinal direction of the radiantsection, wherein the radiant coil is an at least two-pass coil havingone downstream tube and N upstream tubes in two adjacent passes, N beingan integer from 2 to 8; and a manifold connected to an inlet end of thedownstream tube of said at least two-pass coil, and connected to anoutlet end of each of the N upstream tubes of said at least two-passcoil through a curved connector, wherein the curved connector comprisesa U-shaped elbow and an S-shaped elbow, of which one connects to theoutlet end of a corresponding upstream tube and the other connects to aninlet end of the manifold, wherein the N upstream tubes are evenlydivided into a first group and a second group, respectively arranged attwo sides of the downstream tube, and wherein all of the N upstreamtubes and the downstream tube reside in a common plane.
 2. The ethylenecracking furnace according to claim 1, wherein the manifold is in theform of a Y-shaped pipe having N input ends and one output end, N being2 or
 4. 3. The ethylene cracking furnace according to claim 1, whereinviewed from a top view of the radiant coil, with respect to thedownstream tube, corresponding S-shaped elbows are respectively arrangedon two different sides of the common plane, and/or correspondingU-shaped elbows are respectively arranged on two different sides of thecommon plane.
 4. The ethylene cracking furnace according to claim 3,wherein all the S-shaped elbows are in parallel with one another.
 5. Theethylene cracking furnace according to claim 3, wherein all the S-shapedelbows are divided into a plurality of groups, with all S-shaped elbowsin each group in parallel with one another.
 6. The ethylene crackingfurnace according to claim 2, wherein when N equals 4, every twoupstream tubes fluidly connected to the curved connector via oneY-shaped pipe.
 7. The ethylene cracking furnace according to claim 1,wherein the first group of upstream tubes and the second group of theupstream tubes are respectively arranged on two different sides of asecond plane in which the downstream tube resides alone, and wherein thefirst group of upstream tubes is in a mirror relationship with thesecond group of upstream tubes across the second plane.
 8. The ethylenecracking furnace according to claim 7, wherein the first group ofupstream tubes are in a third plane, the second group of upstream tubesare in a fourth plane parallel, both the third plane and the fourthplane are parallel to the second plane.
 9. The ethylene cracking furnaceaccording to claim 1, wherein viewed from a top view of the radiantcoil, the curved connectors of two adjacent upstream tubes arerespectively located on two different sides of the common plane.
 10. Theethylene cracking furnace according to claim 9, wherein each S-shapedelbow is connected to the outlet end of a corresponding upstream tube,and each U-shaped elbow is connected to an inlet end of the manifold,and wherein the S-shaped elbows on either side of the common plane areparallel to each other, and the U-shaped elbows on either side of thecommon plane are parallel to each other.
 11. The ethylene crackingfurnace according to claim 9, wherein each S-shaped elbow is connectedto a corresponding inlet end of the manifold, and each U-shaped elbow isconnected to the outlet end of a corresponding upstream tube, andwherein the S-shaped elbows arranged respectively on two different sidesof the common plane are in a mirror relationship across the commonplane.
 12. The ethylene cracking furnace according to claim 9, whereinthe curved connectors of two adjacent upstream tubes have a same length.13. The ethylene cracking furnace according to claim 1, wherein a tubediameter of the curved connector equals a tube diameter of the upstreamtube.
 14. The ethylene cracking furnace according to claim 1, whereinthe radiant coil is a two-pass coil, the upstream tube being thefirst-pass tube and the downstream tube being the second-pass tube. 15.The ethylene cracking furnace according to claim 1, wherein the radiantcoil has more than two passes, the upstream tubes being used forodd-numbered passes while the downstream tubes being used foreven-numbered passes.